西藏拉萨 - 日多晚侏罗世—早白垩世盆地顺层剪切构造及其地质意义

The bedding shear structures of the Lhasa Riduo basin in the late Jurassic-early Cretaceous, and it’s geological significances

关键词:  拉萨-日多盆地  顺层剪切构造  伸展体制  EBSD

Key words:  Lhasa-Riduo Basin  bedding shear structures  extensional tectonic setting  EBSD

摘要:

拉萨-日多盆地中存在不同构造体制下的两期构造变形。本文运用构造解析法，对盆地中新厘定出的晚侏罗世—早白垩世期间普遍存在的以横向构造置换为主的顺层剪切构造群落进行详细调查研究，划分出顺层剪切作用下形成的顺层韧性剪切带、顺层掩卧褶皱、顺层面理与拉伸线理、粘滞性石香肠及同构造淡色脉等不同构造样式。结合石英显微构造变形及EBSD组构分析，认为顺层剪切构造中石英以中低温组构（400~550℃）的底面和菱面滑移系为主，早期经历了中高温组构（550℃~650℃）的柱面滑移变形，并叠加了后期低温组构的（﹤400℃）底面滑移变形，总体反映了中等及中低温条件下的剪切变形作用。初步建立了拉萨-日多盆地中的顺层剪切构造变形机制的一种可能模式，即伸展体制下大规模的水平分层剪切作用，认为晚侏罗世—早白垩世时期拉萨-日多盆地为伸展盆地。

Abstract:

Located in southern Gangdise volcanic arc, the late Jurassic-early Cretaceous Lhasa-Riduo basin, Tibet, experienced two deformation stages under different tectonic settings. Based on detailed geological investigation, the bedding shear structures such as bedding recumbent folds, bedding foliations, stretching lineations, viscous boudinages and syntectonic leucocratic veins are recognized in this paper. Studies on microstructural deformation and EBSD analyses indicate that deformations of quartzs in bedding shear zones were currently dominated by basal glides and rhombohedral glides,with the mid-low temperature(400℃~500℃)fabrics, but previously, they experienced deformation stages of the medium-high temperature fabric (550℃~650℃) prismatic glide and the late low-temperature fabric (<400℃) basal glide .The authors suggest a possible model for the bedding shear structures in the Lhasa-Riduo Basin. It is believed that the bedding shear structures in the Lhasa-Riduo basin were formed by a large-scale horizontal and stratified shear action which was under the extensional tectonic setting during the Late Jurassic to the Early Cretaceous.